FIG. 7 is a fragmentary plan view of a tape tensioner the present applicant has already disclosed (see U.S. Pat. No. 5,430,585). The drawing shows the tensioner in an unloading state with a magnetic tape T accommodated in a cassette 2. A mechanism deck 100 includes a chassis 1 which has rotatably mounted thereon two reel supports 4 fitting to the respective hubs of the cassette 2. One of the reel supports 4 rotates clockwise during usual playback with small frictional resistance, permitting the tape T to be pulled out under tension. During reverse playback, the reel support 4 rotates counterclockwise to take up the tape T. The take-up torque is transmitted from a known friction mechanism 9 for producing rotational torque by receiving rotation from a drive motor (not shown) to the reel support 4 via a swing idler 90 and an intermediate gear 91.
A tension lever 5 pivoted as at 55 to the chassis 1 at the left of the reel support 4 has a free end carrying an upright tension pin 53 to be in contact with the tape T during running. A tension band 50 reeved around the reel support 4 has opposite ends attached to the tension lever 5 in the vicinity of its pivoted portion.
When the tension lever 5 is positioned in the cassette 2 as loaded in place, a leading guide block 95 for loading the tape T is in a standby position as brought close to the tension lever 5, restraining the lever 5 from rotating counterclockwise inadvertently.
Provided on the rear side of the chassis 1 is a circular-arc control slide 8 rotatable about a phantom point P on the chassis 1. A control lever 7 supported at an intermediate portion thereof by a pivot 75 on the rear side of the chassis 1 has one end engaged in a cam groove 80 formed in the control slide 8. The other end of the control lever 7 extends through the chassis 1 to project toward the same side as the tension lever 5. A tension spring 3 is provided between and engaged with the tension lever 5 and the control lever 7.
FIG. 8 is a plan view showing the tensioner as positioned for usual playback, and FIG. 9 is a plan view showing the same as positioned for reverse playback. In these two running modes, the leading guide block 95 is away from the position of the cassette loaded in place and has completely loaded the tape in position. The difference between the two running modes is that the length of the tension spring 3 stretched by the pivotal movement of the control lever 7 differs.
For usual playback, the control slide 8 rotates counterclockwise about the point P, causing the control lever 7 to stretch the tension spring 3 as shown in FIG. 8. The tension lever 5 rotates counterclockwise, pulling the tension band 50 to offer resistance to the rotation of the reel support 4. At this time, the reel support 4 is in rotation in a direction to pay off the tape T. The tape T is given a tension corresponding to the rotation resisting torque offered to the reel support 4 and divided by the diameter of winding of the tape. The tension pin 53 is in contact with the rear side of the tape T. When the tension on the tape T increases, the tension pin 53 is pushed by the tape T, slackening the tension band 50 and reducing the resistance against the reel support 4 to lessen the tension on the tape. Conversely, if the tension on the tape T diminishes, the tension band 50 is pulled by the tension spring 3, offering greater resistance to the rotation of the reel support 4 and giving an increased tension to the tape.
For reverse playback, the control slide 8 further rotates counterclockwise as shown in FIG. 9. Since the tape T is wound up by the rotation of the reel support 4 during reverse playback, the resistance to the rotation needs to be less than is the case with usual playback. Accordingly, the control lever 7 rotates in such a direction as to reduce the stretch of the tension spring 3, decreasing the tension on the tension band 50. The resistance to the rotation of the reel support 4 becomes smaller than in the usual playback operation.
Thus in the conventional device, the single tension spring 3 gives resistance to the rotation of the reel support 4 during usual playback and reverse playback.
However, the completely rotated positions of the tension lever 5 and the control lever 7 involve variations, which produce variations in the stretched length of the tension spring 3. Accordingly, if the single tension spring is used in the different running modes for giving resistance to the rotation of the reel support 4, the resistance to be offered to the reel support 4 can not be set at accurate values for the respective tape running modes for usual playback and reverse playback, with the result that the force of the tension pin 53 pushing the tape T becomes indefinite, failing to tension the tape with good stability.
If the resistance to rotation increases, the tape take-up torque for reverse playback decreases to result in the likelihood that the tape T will not be wound up. To obviate this drawback, the friction mechanism 9 for giving the take-up torque to the reel support 4 may be altered to a known mechanism to afford higher rotational torque for reverse playback than for usual playback, whereas the friction mechanism 9 then becomes complex in construction.